Water seems to flow freely on Mars

Dark streaks that hint at seasonally flowing water have been spotted near the equator of Mars. The potentially habitable oases are enticing targets for research. But spacecraft will probably have to steer clear of them unless the craft are carefully sterilized; a costly safeguard against interplanetary contamination that may rule out the sites for exploration.
River-like valleys attest to the flow of water on ancient Mars, but today the planet is dry and has an atmosphere that is too thin to support liquid water on the surface for long. However, intriguing clues suggest that water may still run across the surface from time to time.
In 2011, for example, researchers who analysed images from NASA’s Mars Reconnaissance Orbiter (MRO) spacecraft observed dark streaks a few metres wide that appeared and lengthened at the warmest time of the year, then faded in cooler seasons, reappearing in subsequent years. "This behaviour is easy to understand if these are seeps of water," says planetary scientist Alfred McEwen of the University of Arizona in Tucson, who led that study. "Water will darken most soils."
The streaks, known as recurring slope lineae, initially were found at seven sites in Mars’s southern mid-latitudes. The water may have come from ice trapped about a metre below the surface; indeed, the MRO has spotted such ice in fresh impact craters at those latitudes.